Preparation of carboxylic acids by carbonylation of alcohols

ABSTRACT

Carboxylic acids having the structural formula R 1  COOH, wherein R 1  is a linear, branched or cyclic chain alkyl radical of from 1 to 6 carbon atoms or a Φ--C n  H 2n  -- radical wherein Φ is phenyl and 1≦n≦6, are prepared by carbonylating an alcohol having the formula R 1  --OH with carbon monoxide in liquid phase, in the presence of a catalytically effective amount of nickel and an alkyl or acyl halide promoter therefor, at a temperature of at least about 120° C. and under a total pressure of less than 200 bars, and said carbonylation being carried out in the presence of at least one chromium salt in which the chromium is in a state of oxidation (p) of greater than zero (p&gt;0), and in an initial carboxylic acid reaction medium having the formula R 2  --COOH, wherein R 2  is defined as was R 1  above, and further wherein R 1  and R 2  may be the same or different.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation of carboxylic acids, and, more especially, to the preparation of carboxylic acids, notably acetic acid, by carbonylation of an alcohol.

2. Description of the Prior Art

It is well known to this art that, e.g., acetic acid, can be prepared by carbonylation of methanol under relatively severe reaction conditions of pressure, in the presence of nickel and a free or bound halogen.

Thus, it has been proposed, in particular (compare U.S. Pat. No. 2,729,651), to carry out the carbonylation of methanol in the presence of nickel complexes which are obtained by reacting nickel halides with quaternary ammonium (or phosphonium) halides, i.e., complexes of the general formula:

    [A.sub.4 M].sub.2 NiX.sub.4

in which X represents a bromine or iodine atom, M represents a phosphorus or nitrogen atom and A is, for example, a lower alkyl radical.

These complexes can be used in the aforesaid form in the reaction in question, or they can be formed in situ. However, although increased pressure (on the order of 700 atmospheres) is applied during the carbonylation reaction, the efficiency of the catalyst system, expressed in terms of hourly output or yield, is very low.

It has been possible to substantially improve this output, expressed either with respect to the reaction volume or with respect to the amount of nickel used, by employing in the subject reaction a nickel halide, on the one hand, and, on the other hand, a quaternary ammonium (or phosphonium) halide in a quantity greater than that required by the stoichiometry of formation of the complexes of the aforenoted formula. Compare German Pat. No. 933,148. However, in this latter case, the pressure conditions remain severe.

More recently, catalyst systems which permit the carbonylation of methanol under less severe conditions of pressure have been proposed. Thus, published French Patent Application No. 2,370,023 describes the carbonylation of methanol in the presence of at least 10 mols of methyl iodide per 100 mols of methanol, and in the presence of nickel and of a free phosphine and/or a phosphine complexed with the nickel, under a pressure of less than 70 bars. However, the efficiency of such a system, again expressed in terms of hourly output, remains low.

In the context of a carbonylation process under low pressure, it has also been emphasized (compare published French Patent Application No. 2,404,618) that the presence of solvents, such as carboxylic acids or their esters, has a favorable effect on the course of the carbonylation reaction. However, utilization of such solvents is not essential, and the starting material alcohol can also serve as the solvent.

However, the industrial or commercial value of these recent developments is limited as a result of the instability and the cost of the phosphines or of the amines required to effect these techniques.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision of an improved process for the preparation of carboxylic acids, notably alkanoic acids, and especially acetic acid, such improved process being characterized by markedly enhanced output, by carbonylation of alcohols in the presence of nickel and at least one halogen-containing promoter under reaction conditions of relatively mild pressure, and such process being conspicuously devoid of the aforenoted disadvantages and drawbacks.

Briefly, the present invention features an improved process for the carbonylation of alcohols in liquid phase, under a total pressure of less than 200 bars, in the presence of a catalytically effective amount of nickel, an alkyl or acyl halide and at least one chromium salt wherein the chromium is in an oxidation state (p) of greater than zero (p>0), and a beginning amount of a carboxylic acid.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to this invention, it has now surprisingly been found that, e.g., the carbonylation of methanol in acetic acid proceeds quite satisfactorily when the partial pressure of the carbon monoxide is maintained relatively low; this is all the more unexpected since the state of this art has to date clearly pointed to the necessity for carrying out the carbonylation under very high pressures.

Even more particularly, the focus of the present invention is the demonstrated surprising activity of those chromium salts in which the chromium is in an oxidation state (p) greater than zero (p>0), when used as co-catalysts, in the carbonylation of an alcohol, in liquid phase, under a total pressure of less than 200 bars. Indeed, it has even been confirmed that the salts and other derivatives of numerous metals (including metal carbonyls) do not permit the reaction of carbon monoxide with an alcohol in a carboxylic acid medium under the aforesaid reaction conditions, in the presence of nickel and a halogen-containing promoter.

According to the present invention, carbon monoxide is reacted with an alcohol in order to produce the corresponding carboxylic acid, in accordance with the following equation: ##STR1## in which R¹ represents a linear, branched or cyclic alkyl radical containing from 1 to 6 carbon atoms, or a radical Φ--C_(n) H_(2n) --, in which n is an integer ranging from 1 to 6 (1≦n≦6) and Φ is phenyl. The radical R¹ can also bear one or more substituents which are inert under the reaction conditions of the present invention. R¹ is preferably a linear or branched chain lower alkyl radical having from 1 to 4 carbon atoms, and more preferably is a methyl radical.

The process according to the present invention is carried out in the liquid phase in a reaction medium comprising a carboxylic acid of the formula R² COOH, in which R² is defined exactly as was R¹, it thus being possible for R² and R¹ to either be the same or different. Expressed otherwise, the carboxylic acid which, so to speak, functions as a solvent is not necessarily the particular carboxylic acid produced by the carbonylation reaction. However, it may indeed prove preferable for the carboxylic acid employed as the solvent to be that produced in the reaction. Needless to say, employing as the solvent a carboxylic acid which is heavier than the acid produced can facilitate the separation operation.

The carboxylic acid R² COOH advantageously constitutes at least 10% by volume of the initial reaction mixture. It preferably constitutes at least 20% by volume of said reaction mixture. It can constitute a substantial proportion of the reaction mixture, especially in the case of an operation carried out continuously, by injecting the alcohol R¹ OH into the carbonylation reactor.

The process according to the invention requires the presence of a catalytically effective amount of nickel. Any source of nickel can be used within the scope of the present process. Thus, it is possible to introduce the nickel in its metallic form (for example Raney nickel) or in any other convenient form. Examples which are exemplary of nickel compounds which can be utilized for carrying out the present process are: the carbonate, oxide, hydroxide, halides, in particular the iodide, and carboxylates, in particular the acetate, of nickel. Nickel carbonyl is also particularly suitable. Raney nickel, nickel iodide, nickel acetate and nickel carbonyl are preferred to be used.

The amount of nickel is not critical. The proportion of nickel which affects the rate of reaction is determined as a function of the rate of reaction calculated as suitable. In general, an amount of nickel between 5 and 2,000 milligram-atoms per liter of solution gives satisfactory results. The reaction is preferably carried out with an amount of nickel between 20 and 1,000 milligram-atoms per liter.

The process according to the present invention also requires the presence of at least one alkyl or acyl halide. The formulae of these halides are, respectively, R³ X and ##STR2## in which X represents a chlorine or bromine atom, or, preferably, an iodine atom and R³ is defined exactly as was R¹ (and R²), it being possible for R³ and R¹ (or R³ and R²) to be identical or different. The alkyl halide which can initially be used in the reaction mixture can, of course, be formed in situ, starting from halogen derivatives, such as Cl₂, Br₂, I₂, HCl, HBr, HI, NiBr₂ and NiI₂, and from the alcohol (starting material). Stated differently, some or all of the alkyl halide required for carrying out the present process can be formed starting from its "precursors" defined above.

Furthermore, it will be notd that when the halogen derivative is selected from among the nickel compounds, it can be considered not only as a precursor of the alkyl halide but also as a precursor of the metallic catalyst. In this particular case, it is also preferred to add, initially, an alkyl or acyl halide and/or a precursor other than the nickel halides in question.

Within the ambit of the present invention, lower alkyl iodides having from 1 to 4 carbon atoms define a preferred class of alkyl halides. Methyl iodide is particularly suitable for carrying out the process according to the invention.

An alkyl or acyl halide concentration of at least 0.5 mol per liter of reaction mixture is typically required for successfully carrying out the present process. Although increasing the concentration of alkyl or acyl halide has a favorable effect on the rate of reaction, it is preferred not to exceed a concentration on the order of 8 mols per liter. For this reason, a concentration of alkyl or acyl halide between 0.8 and 6 mols/liter, and preferably between 1.5 and 5 mols/liter provides satisfactory results.

One of the essential characteristics of the present invention is the use of at least one chromium salt in which the chromium is in an oxidation state (p) greater than zero (p>0), namely, of at least one compound of the formula Cr_(n) ^(p+) X_(q) ^(m-), in which q denotes the ratio (n×p)/m, m is equal to 1, 2 or 3, p=2, 3, 4 or 6, n being equal to 1 or 2 and being selected in accordance with the respective values of m and p such that q is an integer, and X^(m-) is an anion selected from the group comprising OH⁻, Cl⁻, Br⁻, I⁻, O⁻⁻, PO₄.sup..tbd., CH₃ COCHC(CH₃)O⁻, HCOO⁻, C₂ O₄ ⁻⁻, R⁴ --COO⁻, R⁴ --O⁻, NO₃ ⁻ and CO₃ ⁻⁻, R.sup. 4 being defined exactly as was R¹ and it being possible for R⁴ and R¹ to be either the same or different; the chromium salts can be in hydrated form.

The precise nature of the anion X^(m-) does not appear to be a fundamental parameter of the present process. The following are exemplary of chromium salts which are suitable for carrying out the present process: Cr(OH)₃, CrCl₂, CrCl₃.6H₂ O, CrBr₂, CrBr₃.6H₂ O, CrO₃, Cr₂ O₃, CrPO₄.6H₂ O, Cr(OCOCH₃)₃.H₂ O, Cr(NO₃)₃, CrCO₃, Cr(OCOCH₃)₂, Cr(OH) (HCO₂)₂, Cr[CH₃ COCHC(CH₃)O]₃ and CrC₂ O₄.H₂ O.

It is not necessary to initially introduce a chromium salt in which the chromium is in an oxidation state greater than zero. In fact, it has now been determined that it is possible consistent herewith to introduce chromium, in the zero oxidation state, and an oxidizing agent; this oxidizing agent must not otherwise have an adverse influence on the carbonylation reaction. Thus, the active species of the chromium can be formed in situ from, e.g., chromium hexacarbonyl and iodine. Chromium carboxylates, and more particularly acetates, are convenient to use and in this respect are recommended for carrying out the present invention. According to a preferred embodiment of the invention, a chromium salt in which the chromium is in the oxidation state 3 (p=3) is employed.

Good results are obtained if the atomic ratio Cr/Ni is between 0.1 and 200, although lower or higher ratios can be selected. This ratio is advantageously a value ranging from 0.5 to 100 and preferably ranging from 1 to 50.

If a low Cr/Ni ratio be employed, the activity of the catalyst system defined above can be increased by adding an alkali metal iodide, and more particulary sodium iodide, potassium iodide or lithium iodide, to the reaction medium.

A reaction temperature of at least 120° C. is typically required in order to obtain a satisfactory rate of reaction. A temperature range of from 160° to 200° C. proves advantageous.

According to the present invention, it is not necessary to purify or dry the alcohol and the carboxylic acid initially used in the reaction. Technical grade alcohols and carboxylic acids optionally containing up to 20% by volume of water can be utilized. On the other hand, some or all of the alcohol R¹ OH can be employed in the reaction in the form of an ester R² COOR¹, R¹ and R² being as defined above and it being possible for R¹ and R² to be identical or different. In this case, an amount of water at least equal to the amount theoretically required to hydrolyze the ester initially introduced should also initially be used.

The carbonylation process according to the present invention is carried out in the liquid phase under a pressure greater than atmospheric pressure, the pressure being, however, less than 200 bars. More particularly, it is recommended to carry out the reaction under a partial pressure of carbon monoxide of between 10 and 100 bars. The carbon monoxide is preferably employed in an essentially pure form, such as is commercially available. However, the presence of impurities, such as, for example, carbon dioxide, oxygen, methane and nitrogen, is not harmful. The presence of hydrogen, even in relatively large proportions, likewise is not harmful.

After the reaction, the reaction mixture is separated into it various constituents by any appropriate means, for example, by distillation.

The process according to the invention is particularly suitable for the preparation of acetic acid by carbonylation of methanol, especially in an acetic acid reaction medium.

In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that same are intended only as illustrative and in nowise limitative.

The following symbols and definitions have been used in the said examples:

AcOH designates acetic acid;

AcOMe designates methyl acetate;

RY designates the molar ratio: ##EQU1## i.e., the molar ratio between the "potential acetic acid" formed and the initial (CH₃ OH+CH₃ I);

v designates the initial rate of carbonylation, expressed in mols of carbon monoxide absorbed per hour and per gram-atom of nickel;

t denotes the effective duration of the absorption of the carbon monoxide at the temperature of the experiment;

DC denotes the molar ratio: ##EQU2##

Pr denotes the productivity or output, relative to the time t (expressed in hours), in grams of "potential acetic acid" formed per liter of the initial reaction mixture.

EXAMPLE 1

The following ingredients were introduced into a Hastelloy B 2 autoclave of 125 ml capacity:

(i) 364 mmols of methanol, i.e., 15 ml;

(ii) 341 mmols of acetic acid, i.e., 20 ml;

(iii) 211 mmols of methyl iodide, i.e., 12.5 ml;

(iv) 100 mg atoms of chromium in the form of chromium (III) acetate, i.e., 22.9 g; and

(v) 20 mmols of nickel tetracarbonyl, i.e., 2.6 ml.

After closing the autoclave, a pressure of 40 bars of carbon monoxide was established therein. Shaking by means of a reciprocating system was commenced and the autoclave was heated to 180° C., over the course of about 20 minutes, by means of an annular furnace. The pressure in the autoclave was then 70 bars; it was subsequently maintained at 70 bars by introduction of additional amounts of pure CO.

The pressure drop in the high-pressure supply which continuously fed the autoclave (expansion valve) was recorded.

The absorption of carbon monoxide was complete after a reaction time of 30 minutes at 180° C.; heating was nevertheless continued for an additional one hour and 30 minutes at this temperature.

The shaking and heating were then stopped; the autoclave was cooled and degassed.

After dilution, the resulting reaction mixture was analyzed by gas chromatography. Same contained 52.2 g of acetic acid (RY=92%). Neither methyl acetate nor methanol was detected.

The productivity (Pr) of the reaction in terms of acetic acid was therefore 1,250 grams per hour and per liter (g/hour×liter); the degree of conversion of the methanol was 100%.

EXAMPLES 2 TO 9

Using the apparatus and the procedure described in Example 1, a series of experiments was carried out on a charge comprising 15 ml of methanol, 20 ml of acetic acid, nickel, a chromium salt and methyl iodide. The temperature was 180° C., the pressure was maintained at 70 bars and the duration of the experiment was 2 hours (unless indicated otherwise). The particular conditions and also the results obtained are reported in Table I below.

Control Experiment (a), also reported in the table, was carried out in the absence of a chromium salt.

Control Experiment (b), also reported in this table, was carried out at 190 bars.

                                      TABLE I                                      __________________________________________________________________________     EXAM-    NICKEL                    t     AcOH          RY                                                                                 Pr                  PLE  CH.sub.3 I                                                                         COMPOUND   CHROMIUM SALT  (min- determined                                                                           AcOMe                                                                               DC AcOH                                                                               g/hour              No.  mmols                                                                              type  mg atoms                                                                              type  mg atoms                                                                              utes)                                                                             v  (g)   (g)  %  %   ×             __________________________________________________________________________                                                                liter               1    211 Ni(CO).sub.4                                                                         20     Cr(OAC).sub.3                                                                        100    30 30 52.2  0    100                                                                               92  1250                2    207 "     20     CrI.sub.3                                                                            40     60 30 36.1  0    88 46  ND                  3    208 Ni(OAc).sub.2                                                                        8      CrO.sub.3                                                                            80     ND 62.5                                                                              ND         95 ND  ND                  4    205 "     10     Cr(CO).sub.6                                                                                120                                                                                6 17.7  7.4  ND 10  ND                  5    206 "     8      Cr(OAc).sub.3                                                                        150    40 87.5                                                                              48.7  0    100                                                                               86   860                6    206 "     8      "     50     75 56 45.1  0    100                                                                               74  410                 7    203 "     8      "     10     120                                                                               19 24.9  8.66 60 34  115                 8x   209 Ni(CO).sub.4                                                                         10     "     10     ND 35 43.7  0    100                                                                               70  ND                  9xx  209 "     10     "     10     70 45 41    0    100                                                                               60  400                 a    249 "     20           0      120                                                                                0 10.5  10.6 0   0   0                  b    199 Ni(OAc).sub.2                                                                        8      Cr(OAc).sub.3                                                                        100    ND ND 19.4  12.3 36 24   80                 __________________________________________________________________________      NB: in this table, Ni(OAc).sub.2 denotes nickel acetate tetrahydrate.          ND: not determined                                                             x: this experiment was carried out with the introduction of 100 mmols of       NaI                                                                            xx: this experiment was carried out with the introduction of 50 mmols of       KI                                                                       

CONTROL EXPERIMENTS (C) TO (N)

Using the equipment and the procedure described in Example 1, a series of experiments was carried out on a charge comprising 15 ml of methanol, 20 ml of acetic acid and methyl iodide, at 180° C. and under 70 bars of pressure, in the presence of various metal compounds. No reaction was observed after 2 hours at the temperature indicated.

The particular conditions of these control experiments are reported in Table II below:

                                      TABLE II                                     __________________________________________________________________________                           CH.sub.3 I                                                       NICKEL COMPOUND                                                                              in  METAL COMPOUND                                       REFERENCE                                                                              type     mg atoms                                                                            mmols                                                                              type     mg atoms                                    __________________________________________________________________________     c       Ni(CO).sub.4                                                                            20   201 Cr(CO).sub.6                                                                            50                                          d       Ni(CO).sub.4                                                                            20   206 Zn(OAc).sub.2.2H.sub.2 O                                                                100                                         e       Ni(CO).sub.4                                                                            20   203 Zn       50                                          f       Ni(CO).sub.4                                                                            20   209 Mn(OAc).sub.2.4H.sub.2 O                                                                100                                         g       Ni(CO).sub.4                                                                            20   204 AlI.sub.3                                                                               50                                          h       Ni(CO).sub.4                                                                            20   203 Co(OAc).sub.2.4H.sub.2 O                                                                100                                         i       Ni(OAc).sub.2.4H.sub.2 O                                                                10   205 MoBr.sub.3                                                                              30                                          j       Ni(OAc).sub.2.4H.sub.2 O                                                                 8   202 Fe(OAc).sub.2                                                                           100                                         k       Ni(CO).sub.4                                                                            20   205 MoI.sub.3                                                                               10                                          l       Ni(CO).sub.4                                                                            10   209 WI.sub.3 10                                          m       Ni(CO).sub.4                                                                            20   211 Mg(OAc).sub.2.4H.sub.2 O                                                                100                                         n       Ni(CO).sub.4                                                                             8   200 BiI.sub.3                                                                               40                                          __________________________________________________________________________

EXAMPLES 10 TO 14

Using the equipment and the procedure described in Example 1, a series of experiments was carried out on a charge comprising methanol, acetic acid, methyl iodide, 8 mg atoms of nickel in the form of nickel acetate tetrahydrate and 100 mg atoms of chromium in the form of chromium(III) acetate. The temperature was 180° C.; the pressure was maintained at 70 bars (unless indicated otherwise) by introduction of additional amounts of pure CO. The duration of each experiment was 2 hours. The particular conditions and the results obtained are reported in Table III below:

                                      TABLE III                                    __________________________________________________________________________                                    AcOH          Pr                                       CH.sub.3 OH                                                                         AcOH                                                                               CH.sub.3 I                                                                         DC t       determined                                                                           AcOMe                                                                               RY g/hour                            Example No.                                                                           ml   ml  mmols                                                                              %  minutes                                                                             v  (g)   (g)  %  × liter                     __________________________________________________________________________     10     19   26   40 12 ND   <6 13.4  18.4  6 ND                                11     18   24   80 44 120  17.5                                                                              25.9  12.9 39 ND                                12     16   22  161 100                                                                               75   50 50.2  0    85 450                               13     15   20  205 100                                                                               50   69 46.8  0    80 600                                14x   15   20  204 98 ND   31 48    0    77 270                               __________________________________________________________________________      x: experiment carried out under a total pressure of 40 bars              

EXAMPLES 15 TO 17

Using the equipment and the procedure described for Example 1, a series of experiments was carried out on a charge comprising 5 ml of water, methyl acetate, acetic acid, 8 mg atoms of nickel in the form of nickel acetate tetrahydrate, 80 mg atoms of chromium in the form of chromium(III) acetate, and methyl iodide. The temperature was 180° C.; the total pressure was maintained at 70 bars by introduction of additional amounts of pure CO. The duration of each experiment was 2 hours. The particular conditions and also the results obtained are reported in Table IV below:

                                      TABLE IV                                     __________________________________________________________________________                                    AcOH         Pr                                        AcOMe                                                                               AcOH                                                                               CH.sub.3 I                                                                         DC t      determined                                                                           AcOMe                                                                               RY g/hour                             Example No.                                                                           ml   ml  mmols                                                                              %  minutes                                                                             v (g)   (g)  %  × liter                      __________________________________________________________________________     15     20   20  84  71 120  19                                                                               42.6  5.4  60 120                                16     10   30  85  100                                                                               100  14                                                                               49    0    84 125                                17     30   10  84  55 120  20                                                                               34.8  11   42 110                                __________________________________________________________________________

EXAMPLE 18

Using the autoclave and the procedure described for Example 1, 15 ml of methanol, 20 ml of acetic acid, 162 mmols of methyl iodide, 8 mg atoms of nickel in the form of the acetate tetrahydrate and 80 mg atoms of chromium in the form of chromium(III) acetate were introduced into the autoclave. The reaction temperature was 160° C. The total pressure was maintained at 70 bars by introduction of additional amounts of pure CO. After a reaction time of two hours at the temperature indicated, 24.8 g of acetic acid and 11.1 g of methyl acetate were obtained. (Pr=130 g/hour×liter).

EXAMPLE 19

Example 18 was repeated, but at a temperature of 200° C. The absorption of carbon monoxide was complete after a reaction time of 20 minutes at the indicated temperature; heating was nevertheless continued for an additional one hour, 40 minutes at this temperature. 50.5 g of acetic acid were determined by analysis. The mixture contained neither methyl acetate nor methanol. (Pr=1,760 g/hour×liter).

EXAMPLE 20

Using the equipment and the procedure described in Example 1, 15 ml of methanol, 20 ml of acetic acid, 3.5 ml of water, 21 mg atoms of nickel in the form of nickel tetracarbonyl, 205 mmols of methyl idodide and 100 mg atoms of chromium in the form of chromium(III) acetate were introduced into the autoclave. The temperature was 180° C.; the total pressure was maintained at 70 bars by introduction of additional amounts of pure CO. The duration of the experiment was 2 hours at this temperature, although the absorption of carbon monoxide was complete after 30 minutes.

Upon completion of the experiment, 45.5 g of acetic acid were determined by analysis. The reaction medium contained neither methyl acetate nor methanol (Pr=1,000 g/hour×liter).

EXAMPLE 21

Using the equipment and the procedure described in Example 1, 25 ml of methanol, 25 ml of acetic acid, 8 mg atoms of nickel in the form of the acetate tetrahydrate, 100 mmols of iodine and 80 mg atoms of chromium in the form of chromium(III) acetate were introduced into the autoclave. The temperature was 180° C. The reaction was carried out under a partial pressure of hydrogen of 30 bars, the total pressure being maintained at 90 bars by introduction of additional amounts of pure CO. The duration of the experiment was 3 hours. Upon completion of the experiment, 55.6 g of acetic acid were determined by analysis. The reaction mixture contained neither methyl acetate nor methanol. (RY=83%).

EXAMPLE 22

The following were introduced into a titanium autoclave of 225 ml capacity:

(i) 755 mmols of methanol, i.e., 30 ml;

(ii) 692 mmols of acetic acid, i.e., 40 ml;

(iii) 415 mmols of methyl iodide, i.e., 25 ml ;

(iv) 200 mg atoms of chromium in the form of chromium(III) acetate, i.e., 46 g; and

(v) 16 mg atoms of nickel in the form of nickel acetate tetrahydrate, i.e., 4 g.

After closing the autoclave, a pressure of 30 bars of carbon monoxide was established therein. Shaking by means of a reciprocating system was begun and the autoclave was heated to 180° C., over the course of about 30 minutes, by means of an annular furnace. The pressure in the autoclave was then 65 bars; same was subsequently maintained at 70 bars by introduction of additional amounts of pure CO.

The pressure drop in the high-pressure supply which continuously fed the autoclave (expansion valve) was recorded.

The absorption of carbon monoxide was complete after a reaction time of 40 minutes at 180° C.; heating was nevertheless continued for an additional 80 minutes.

The shaking and heating were then terminated; the autoclave was cooled and degassed.

The resulting reaction mixture was analyzed by gas chromatography. Same contained 83.3 g of acetic acid (RY=60%). Neither methanol nor methyl acetate was detected (DC=100%).

The productivity of the reaction in terms of acetic acid was therefore 620 g/hour×liter. The initial carbonylation rate was 130 mols/hour×g atom (Ni).

EXAMPLES 23 TO 25

A series of experiments was carried out on a charge comprising 30 ml of methanol, 40 ml of acetic acid, nickel acetate tetrahydrate (unless indicated otherwise), methyl iodide and 200 mg atoms of chromium in the form of chromium(III) acetate.

The temperature was 180° C.; the pressure was maintained at 70 bars by introduction of additional amounts of pure CO, and the duration of each experiment was 2 hours.

The particular conditions and also the results obtained are reported in Table V below.

Control Experiment o was carried out in the absence of nickel.

                  TABLE V                                                          ______________________________________                                                Ni                                  Pr                                  Example                                                                               mg      CH.sub.3 I                                                                             t             DC    g/hour                              No.    atoms   mmols   minutes v     %     × liter                       ______________________________________                                         22     16      415     40      130   100   620                                 23     8       415     90      87    100   250                                 24     4       413     120     75     73   160                                 25.sup.+                                                                              40      403     25      62.5  100   1400                                control o                                                                             0       420             0      0     0                                  ______________________________________                                          .sup.+ : experiment carried out with nickel tetracarbonyl                

While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims. 

What is claimed is:
 1. A process for the preparation of a carboxylic acid having the structural formula R¹ COOH, wherein R¹ is a linear, branched or cyclic chain alkyl radical of from 1 to 6 carbon atoms or a Φ--C_(n) H_(2n) -- radical wherein Φ is phenyl and 1≦n≦6, comprising carbonylating an alcohol having the formula R¹ --OH with carbon monoxide in liquid phase, in the presence of a catalytically effective amount of nickel and an alkyl or acyl halide promoter therefor, wherein said acyl halide is of the formula ##STR3## in which X represents a chlorine, bromine or iodine atom and R³ is defined as was R¹ above, and further wherein R³ and R¹ may be the same or different, and under a total pressure of less than 200 bars, said carbonylation being carried out in the presence of at least one chromium salt in which the chromium is in an oxidation state (p) of greater than zero (p>0), and in an initial carboxylic acid reaction medium having the formula R² --COOH, wherein R² is defined as was R¹ above, and further wherein R¹ and R² may be the same or different.
 2. The process as defined in claim 1, wherein the alkyl or acyl halide is an iodide.
 3. The process as defined by claim 2, wherein the halide is a C₁ -C₄ -alkyl iodide.
 4. The process as defined by claim 3, wherein the alkyl iodide is methyl iodide.
 5. The process as defined by any one of claims 1 to 4, wherein R¹ is a C₁ -C₄ -alkyl radical and R² is a linear, branched or cyclic chain C₁ -C₆ -alkyl radical or a Φ--C_(n) H_(2n) -- radical, in which Φ is phenyl and 1≦n≦6.
 6. The process as defined by claim 5, wherein R¹ is methyl.
 7. The process as defined by claim 1, wherein the carboxylic acid R² COOH comprises at least 10% by volume of the initial reaction medium.
 8. The process as defined by claim 7, wherein the concentration of nickel in the reaction medium ranges from 5 to 2,000 mg-atoms per liter.
 9. The process as defined by claim 8, wherein said concentration of nickel ranges from 20 to 1,000 mg-atoms per liter of reaction medium.
 10. The process as defined by claim 8, wherein the concentration of alkyl or acyl halide in the reaction medium ranges from 0.5 to 8 mols per liter.
 11. The process as defined by claim 10, wherein said concentration of alkyl or acyl halide ranges from 0.8 to 6 mols per liter.
 12. The process as defined by claim 1 or 10, wherein R¹ and R² are identical.
 13. The process as defined by claim 12, wherein the chromium salt has the formula Cr_(n) ^(p+) X_(q) ^(m-), in which q is the ratio (n×p)/m, m is equal to 1, 2 or 3, p=2, 3, 4, or 6, n is 1 or 2 and is selected in accordance with the respective values of m and p such that q is an integer, and X^(m-) is OH⁻, Cl⁻, Br⁻, I⁻, O⁻⁻, PO₄.sup..tbd., CH₃ COCHC(CH₃)O⁻, HCOO⁻, C₂ O₄ ⁻⁻, R⁴ --COO⁻, R⁴ --O⁻, NO₃ ⁻ and CO₃ ⁻⁻, R⁴ being defined exactly as was R¹ and R⁴ and R¹ may be the same or different.
 14. The process as defined by claim 13, wherein p=3.
 15. The process as defined by claim 13, wherein the anion (X^(m-)) is a carboxylate of the formula R⁴ --COO⁻ in which R⁴ is defined as was R¹, and R⁴ and R¹ may be the same or different.
 16. The process as defined by claim 15, wherein the anion (X^(m-)) is acetate.
 17. The process as defined by claim 13, wherein the atomic ratio Cr/Ni ranges from 0.1 to
 200. 18. The process as defined by claim 17, wherein the ratio Cr/Ni ranges from 0.5 to
 100. 19. The process as defined by claim 18, wherein the ratio Cr/Ni ranges from 1 to
 50. 20. The process as defined by claim 1, wherein the reaction temperature is at least 120° C.
 21. The process as defined by claim 20, wherein the reaction temperature ranges from 160° to 200° C.
 22. The process as defined by claim 21, wherein the partial pressure of the carbon monoxide ranges from 10 to 100 bars. 